In the United States and many other countries, heart disease is the leading cause of death and disability. One particular kind of heart disease is atherosclerosis, which involves the degeneration of the walls and lumen of the artery walls throughout the body. Scientific studies have demonstrated the thickening of the arterial wall and eventual encroachment of the tissue into the lumen as fatty material is built up. This material is known as "plaque." As the plaque builds up and the lumen narrows, blood flow is restricted. If the artery narrows too much, or if a blood clot forms at an injured plaque site (lesion), flow is severely reduced, or cut off and consequently the muscle that it supports may be injured or die due to a lack of oxygen. Atherosclerosis can occur throughout the human body, but it is most life threatening when it involves the coronary arteries which supply oxygen to the heart muscles. If blood flow to the heart muscle is significantly reduced or cut off, a myocardial infarction or "heart attack" often occurs. If not treated in sufficient time, a heart attack frequently leads to death.
The medical profession relies upon a wide variety of tools to treat coronary disease, ranging from drugs to open heart "bypass" surgery. Often, a lesion can be diagnosed and treated with minimal intervention through the use of catheter-based tools that are threaded into the coronary arteries via the femoral artery in the groin. For example, one treatment for lesions is a procedure known as percutaneous transluminal coronary angioplasty (PTCA) whereby a catheter with an expandable balloon at its tip is threaded into the lesion and inflated. The underlying lesion is re-shaped, and hopefully, the lumen diameter is increased to restore blood flow.
The practiced method for guiding a catheter during the performance of procedures such as PTCA has been to use real time X-ray images. With this method, a radiopaque dye is injected into the coronary tree in order to provide a map of blood flow. This technique facilitates identification by a physician of sites where blood flow is restricted. After identifying the sites, therapeutic devices are positioned using a live X-ray image for guidance in order to treat the lesion(s). However, the X-ray image does not give information about the morphology, i.e., form and structure, of the artery.
In the last 5 years, cardiologists have adopted a new technique to obtain information about the coronary vessel and to help view the effects of the therapy on the form and structure of the vessel and not just the blood flow. This technique, known as Intracoronary or Intravascular Ultrasound (ICUS/IVUS) employs miniaturized transducers on the tip of the catheter which provide electronic signals to an external imaging system in order to produce a two or three-dimensional image of the lumen, the arterial tissue, and tissue surrounding the artery. These images are generated in substantially real time and have a high degree of resolution. As an improvement over X-ray imaging, the transducers facilitate the construction of images of the exact site where the transducers are placed within the vessel.
Several ICUS/IVUS devices are now commercially available for sale in the United States and other countries. These devices include a transducer probe assembly having either a solid state transducer array or a rotating crystal. The physician is most interested in identifying the size and shape of the lumen, and any flaps or tears in the plaque, and these commercially available imaging devices facilitate the creation of detailed images of these relatively static features due to the relatively high frequency of ultrasound that they employ. Image signals are typically transmitted at frequencies between 10 and 40 MHz.
However, there is a common problem associated with these devices operating at such high frequencies. As the frequency of the ultrasound is raised, the backscatter from blood increases as the fourth power of the frequency. At frequencies of around 30 MHz, the amplitude of the backscatter from blood approaches the amplitude of the backscatter and reflections from the arterial tissue. Because of this phenomenon, the image of the lumen is filled with blood echoes, and it is often difficult to delineate the blood from the surrounding tissue. Therefore, this becomes confusing to the physician who is interested in defining the lumen.
A common method of detecting blood flow in ultrasonic systems used outside of the body is the use of a "Doppler" technique. The Doppler technique involves the detection of a change in frequency of a wave due to the reflection of the wave from a moving target. This technique is well established in radar literature such as M. Skolnik: "Introduction to Radar Systems", Second Edition, 1980. The Doppler technique, and variations of it, have been successfully applied to ultrasonic scanners used outside the body to provide color overlay maps of flow on top of grey scale images. A number of commercial systems utilizing this Doppler imaging technique are available, and are well known to those familiar with the state of the art.
However, the Doppler technique has its limitations when applied to arterial imaging. The Doppler technique relies upon the existence of a component of flow toward or away from the direction of the ultrasonic beam emitted by the transducer. In the case of cross-sectional arterial imaging, there is little or no component of flow to which the Doppler effect can be applied since substantially all flow is in a direction orthogonal to the ultrasonic beam.
A technique is known which attempts to extract a flow image from pixel data for a sequence of whole frame video images containing both flow and static portions. In this technique, pixel data for several whole frame video images are obtained over a period of seconds. In order to gather the data for each of the whole frame video images, an ultrasound transducer assembly transmits and receives a series of signals from all radial regions of the imaged volume in the vicinity of the transducer assembly. It is important to note that in gathering the data for the pixel data for a single whole frame video image, no two transduced echo signals in the set of received echo signals used to create the single whole frame video image are received from the same radial region of the imaged volume.
In this imaging technique, the process of gathering data for a single whole frame video image is repeated several times over a period of time of more than one second in order to obtain pixel data for a series of whole frame video images from which a single combined video image is to be created. Thereafter, the differences between values for corresponding pixel points within successive whole frame video images are averaged in an attempt to create a single frame image based upon the pixel data from the series of whole frame images. By averaging the differences between corresponding pixel data between frames, the resulting image is characterized by attenuation of features of the image that remain motionless for the entire frame gathering procedure which lasts on the order of more than one second. This is entirely unacceptable when one attempts to image the relatively dynamic vessels near the heart.
The above described technique, involving the comparison of the data from sequentially created whole frame images, represents an attempt to provide an image of dynamic features in a field of view containing both static and dynamic features. However, this imaging technique contains certain inherent limitations which reduce the utility of this imaging technique when applied to living vascular imaging in organisms. First, it takes more than a second (or even several seconds) to obtain a sufficient number of whole frame images to carry out the comparison and averaging of corresponding pixel values. Second, in a pulsatile artery, the vessel wall and moving intimal flaps are not motionless over a period of a second and therefore will not cancel out when the pixel values for corresponding positions in the whole frame images are compared. Third, cross-sections of a vessel in which blood flow stagnates provide a relatively static signal and therefore may be canceled out along with the rest of the other static portions of the image.
Additionally, it should be noted that the coronary tree, which comprises the vessels of primary interest to cardiologists, is the most rapidly moving vessel structure within the human body. When ultrasonic images of coronary arteries are made, the position of the tissue constantly changes during the data acquisition period due to the influence of the heart cycle upon the imaged tissue. Consequently, the image created by the dynamic vascular tissue will blend with the blood flow image if the above whole frame comparison technique is employed.
Furthermore, the relatively long data acquisition time required for the prior known technique prevents visual reproduction of the potentially useful dynamic information present in pulsatile flow.